1. Field of the Invention
The invention relates to a diagnostic apparatus and diagnostic method for a belt squeezing force adjusting mechanism in a continuously variable transmission in which a belt is wound around a primary sheave that receives driving force and a secondary sheave that outputs force to a drive-line, and which continuously (i.e., in a non-stepped manner) changes the output rotation speed by adjusting the width of the groove of the primary sheave and the secondary sheave by shift control.
2. Description of the Related Art
Japanese Patent Application Publication No. 60-157553 (JP-A-60-157553) and Japanese Patent Application Publication No. 2006-248371 (JP-A-2006-248371), for example, describe apparatuses for diagnosing a malfunction in a continuously variable transmission. Because it is important that electromagnetic valves used to adjust the position of a primary sheave function properly in order to control the speed ratio of the continuously variable transmission, the technology described in JP-A-60-157553 diagnoses a malfunction in two electromagnetic valves in a shift adjusting apparatus that adjusts the position of the primary sheave. Similarly, the technology described in JP-A-2006-248371 diagnoses whether a duty solenoid used to adjust the position of the primary sheave is disconnected.
Moreover, in a continuously variable transmission, the squeezing force is adjusted by a belt squeezing force adjusting mechanism so that the belt will not slip on the sheave. A diagnostic must be performed on this belt squeezing force adjusting mechanism to keep the continuously variable transmission functioning properly.
That is, if the belt squeezing force adjusting mechanism is not functioning properly due to a solenoid valve being disconnected or the like, and as a result, the belt is squeezed more than necessary, the continuously variable transmission may not be able to upshift sufficiently when the primary sheave side attempts to reduce the speed ratio, i.e., when the primary sheave side attempts an upshift, if the squeezing force on the secondary sheave side remains strong. Therefore, it is necessary to determine whether the belt squeezing force adjusting mechanism is functioning normally or is malfunctioning.
In JP-A-60-157553 and JP-A-2006-248371 described above, a diagnosis that an electromagnetic valve, which adjusts the speed ratio on the primary sheave side, is malfunctioning is made when the actual sheave position is unable to come close to a target sheave position when the electromagnetic valve is being controlled to perform upshift, i.e., when the actual speed ratio is unable to come close to a target speed ratio.
Because a malfunction in the belt squeezing force adjusting mechanism also makes it difficult to change the speed ratio, the diagnostic may be the same as it is in JP-A-60-157553 and JP-A-2006-248371. However, if the diagnostic is performed under the same conditions as those for determining a malfunction in the speed ratio adjustment on the primary sheave side in this way, speed ratio control may appear normal due to the fact that the primary sheave side is able to be adjusted, even though there may actually be a malfunction in the belt squeezing force adjusting mechanism. In this case, there may be an erroneous diagnosis that the belt squeezing force adjusting mechanism is functioning normally even though it is not.